An image forming method by thermal transfer system has been attempted, for example, for printing out an image as taken with an electronic still camera or the like, to a photographic paper like silver salt photography.
In accordance with the thermal transfer system of the type, an ink ribbon containing dyes is brought into contact with a photographic paper having an image-receiving layer as formed thereon and the ink ribbon is heated with a thermal head or the like so that the dyes in the ink ribbon are transferred to the image-receiving layer of the paper. As the image-receiving layer of a photographic paper, used is a polyester; and as the dyes to be in an ink ribbon, used are disperse dyes.
Various reformations and modifications have heretofore been applied to disperse dyes which are generally used in the system of the kind, for example, those disclosed in JP-A 1-259989 and 1-275096 (the term "JP-A" as used herein means an "unexamined published Japanese patent application"). However, after transferred to an image-receiving layer, they are retained therein by mere interaction between the transferred dyes and the layer, for example, by van der Waals force of the former to the high polymer substance of the latter, interamphoteric force or hydrogen bond therebetween or the like. Therefore, where the layer to which dyes have been transferred is brought into contact with a resin or solvent having a higher affinity to the dyes or where heat energy enough to dismiss the interaction is imparted to the layer, migration of dissolution of the transferred dyes would thereby be induced or the image formed would become blurred, disadvantageously.
Under the situation, other means based on formation of chemical bond between transferred dyes and an image-receiving layer have been proposed, for example, in JP-A 59-78893, 60-2398, 60-110494, 60-220785, 60-260381 and 60-260391.
The methods disclosed in the laid-open specifications are a method of incorporating a compound having epoxy groups or isocyanate groups into an image-receiving layer to which dyes having groups capable of reacting with the epoxy or isocyanate groups are applied; a method of incorporating a compound having active hydrogens to an image-receiving layer to which dyes having acryloyl groups or methacryloyl groups are applied; a method of incorporating a metal compound into an image-receiving layer to which dyes capable of forming metal complexes with the metal compound are applied; and a method of subliming and transferring a low molecular compound having an active methyl group (or methylene group) to an image-receiving layer followed by reacting the compound with an aldehyde (nitroso) compound to form a dye in the layer.
However, the means based on formation of chemical bond have various drawbacks that the reactivity of the dyes and the image-receiving layer is too high so that the storage stability of them is poor, that the reaction is not finished in a short period of time and therefore a long time is needed for forming a stable image, and that preparation of the dyes is difficult and the kind of usable dyes is limited. In addition, even by the means, fixation of the transferred dyes could hardly be said to be sufficient.
As mentioned above, the conventional thermal transfer system has a serious problem of migration of the transferred dyes due to insufficiency of the fixability of them on the image-receiving layer, which is a bar to practical operation of the system. Therefore, improvement of the point is desired.